[1] With a hybrid work vehicle in which a motor generator assists an internal combustion engine, the vehicle travels with power from the internal combustion engine, and when the internal combustion engine is under a heavy load, the motor generator is operated as a motor and vehicle travel is assisted by power that is output from this motor generator. The motor generator is also operated as a generator to charge the battery.
For example, a hybrid power unit equipped with an electric motor that provides torque assist to the internal combustion engine when the vehicle starts travelling and is accelerating according to Patent Document 1 detects the charge state of the battery, computes the amount of auxiliary torque (assist amount) that can be supplied to the internal combustion engine from the electric motor based on the detected charge state, and changes the load ratio of the electric motor and the amount of fuel that is provided to the internal combustion engine based on the computed amount of auxiliary torque. Battery exhaustion is thereby prevented by stopping supply of electricity to the electric motor if the charging rate is low.
With a hybrid power unit according to Patent Document 2, two control maps having different assist patterns (relationship between the number of engine revolutions and torque) of the motor generator assisting the internal combustion engine are prepared, and assist control is performed by switching the control maps depending on state information such as the state of charge (SOC) of the battery, vehicle speed, the state of the transmission and the temperature of the internal combustion engine. The aim is to thereby realize favorable operability using a small horsepower internal combustion engine.
[2] With a typical hybrid car in which a motor generator assists an internal combustion engine, the vehicle travels with power from the internal combustion engine, and the motor generator is operated as a motor according to driving conditions (vehicle speed, operation amount of accelerator (accelerator position), operational state of internal combustion engine, road surface conditions, shift position, remaining battery charge, etc.) to assist vehicle travel with power that is output from the motor generator. The motor generator can also be operated as a generator, thus also enabling the battery to be supplied with electricity and charged. This hybrid vehicle is constituted such that when the motor generator is operated as a motor, torque that the motor generator is responsible for providing (assist torque) can be produced according to the load ratio (derived based on driving conditions, etc.) of the internal combustion engine and the motor generator relative to the vehicle drive torque required by the driver (target vehicle drive torque; derived, for example, based on the accelerator pedal operation by the driver) by controlling the torque produced by the motor generator.
The hybrid power unit equipped with an electric motor for providing torque assist to the internal combustion engine when the vehicle starts travelling and is accelerating according to Patent Document 1 detects the charge state of the battery, computes the amount of auxiliary torque (assist amount) that can be supplied to the internal combustion engine from the electric motor based on the detected charge state, and changes the load ratio of the electric motor and the amount of fuel that is provided to the internal combustion engine based on the computed amount of auxiliary torque. Battery exhaustion is thereby prevented by stopping supply of electricity to the electric motor if the charging rate is low.
With the hybrid power unit according to Patent Document 2, two control maps having different assist patterns (relationship between the number of engine revolutions and torque) of the motor generator assisting the internal combustion engine are prepared, and assist control is performed by switching the control maps depending on state information such as the state of charge (SOC) of the battery, vehicle speed, the state of the transmission and the temperature of the internal combustion engine. Favorable operability is thereby realized using a small horsepower internal combustion engine.
[3] With vehicles such as described above, it is possible to keep the speed of the vehicle constant both at high and low engine revolutions through cooperative control of an engine control unit and a shift control unit. For example, Patent Document 3 describes a mobile agricultural machine that is provided with a shift actuator for operating a continuously variable shift mechanism and changes travel speed (the speed of the vehicle) steplessly by controlling the shift actuator. With this mobile agricultural machine, an accelerator actuator and a rotation sensor for detecting and adjusting engine revolutions are provided, and the shift ratio of the continuously variable shift mechanism and the engine revolutions are correlatively (cooperatively) controlled by operating these actuators so as to achieve a prescribed travel speed. The intent is to run the vehicle economically by setting the engine to revolutions at which the fuel consumption rate is low when travelling under a light load, and to run the vehicle to maintain a prescribed speed by setting the engine to high output when travelling under a heavy load.
The ability to reduce engine revolutions while keeping the speed of vehicle constant is advantageous in terms of lowering energy consumption (hereinafter, referred to as energy saving) as a result of the reduced fuel consumption rate. However, a problem arises in that the engine is more likely to stall when engine torque approaches its maximum, causing the vehicle to lose travel stability. Whether or not there is room to increase engine torque depends on the travel state of the vehicle, that is, on road conditions, work conditions and the like, for example. Travelling up a steep incline or along a muddy road naturally results in engine torque approaching its maximum. While the driver is able to get a grasp of the situation, the vehicle according to Patent Document 3 is not provided with a shift operation system that enables the driver's grasp of the situation to be tied in with energy saving driving as referred to above.
A vehicle that realizes shift operation that allows the driver's feel for the excess capacity of the engine to be applied to energy saving driving is disclosed in Patent Document 4. With this vehicle, a revolution reduction instruction for reducing engine revolutions set by the engine control unit by a prescribed amount based on an operation instruction sent out as a result of an operation by the driver is given to the engine control unit, and a shift ratio change instruction for requesting the shift control unit to change the shift ratio to compensate for the reduction in engine revolutions resulting from the revolution reduction instruction is given to the shift control unit in order to maintain the speed of the vehicle. Accordingly, with this well-known vehicle, a revolution reduction instruction for reducing engine revolutions by a prescribed amount can be given to the engine control unit, by the driver operating an operation device when he or she feels that there is room to increase engine torque and wants to reduce engine revolutions for the purpose of energy saving driving or the like. Moreover, the shift ratio is changed commensurate with the reduced engine revolutions and the speed of the vehicle is maintained. In other words, driving that reduces engine revolutions while maintaining the speed of the vehicle is realized, simply by operating an operation device when the vehicle is cruising. However, engine torque approaches its maximum when engine revolutions are excessively reduced while maintaining the speed of the vehicle, causing the vehicle to lose travel stability with a slight increase in engine load and giving rise to the risk of the engine stalling, and it thus becomes necessary to revert to the original number of engine revolutions. Particularly in the case of an inexperience driver, the problem arises of the operations for reducing and increasing engine revolutions being repeatedly performed.
[4] With work vehicles such as described above, it is possible to keep the speed of the work vehicle constant both at high and low engine revolutions through cooperative control of an engine control unit and a shift control unit. For example, Patent Document 3 describes a mobile agricultural machine that is provided with a shift actuator for operating a continuously variable shift mechanism and changes travel speed (the speed of the work vehicle) steplessly by controlling the shift actuator. With this mobile agricultural machine, an accelerator actuator and a rotation sensor for detecting and adjusting engine revolutions are provided, and the shift ratio of the continuously variable shift mechanism and the engine revolutions are correlatively (cooperatively) controlled by operating these actuators so as to achieve a prescribed travel speed. The intent is to run the work vehicle economically by setting the engine to revolutions at which the fuel consumption rate is low when travelling under a light load, and to run the work vehicle to maintain a prescribed speed by setting the engine to high output when travelling under a heavy load.
The ability to reduce engine revolutions while keeping the speed of the work vehicle constant is advantageous in terms of lowering energy consumption (hereinafter, referred to as energy saving) as a result of the reduced fuel consumption rate. However, a problem arises in that the engine is more likely to stall when engine torque approaches its maximum, causing the work vehicle to lose travel stability. Also, if engine revolutions are low, the rotations of a hydraulic pump that is driven by power from the engine will also be low, resulting in a reduction in the amount of operating oil supplied by the hydraulic pump.
A work vehicle that realizes shift operation that allows the driver's feel for the remaining capacity of the engine to be applied to energy saving driving is disclosed in Patent Document 4. With this work vehicle, a revolution reduction instruction for reducing engine revolutions set by the engine control unit by a prescribed amount based on an operation instruction sent out as a result of an operation by the driver is given to the engine control unit, and a shift ratio change instruction for requesting the shift control unit to change the shift ratio to compensate for the reduction in engine revolutions resulting from the revolution reduction instruction is given to the shift control unit in order to maintain the speed of the work vehicle. Accordingly, with this well-known work vehicle, a revolution reduction instruction for reducing engine revolutions by a prescribed amount can be given to the engine control unit, by the driver operating an operation device when he or she feels that there is room to increase engine torque and wants to reduce engine revolutions for the purpose of energy saving driving or the like. Moreover, the shift ratio is changed commensurate with the reduced engine revolutions and the speed of the work vehicle is maintained. In other words, driving that reduces engine revolutions while maintaining the speed of the vehicle is realized, simply by operating an operation device when the vehicle is cruising. However, engine torque approaches its maximum when engine revolutions are excessively reduced while maintaining the speed of the work vehicle, causing the vehicle to lose travel stability with a slight increase in engine load and giving rise to the risk of the engine stalling, and it thus becomes necessary to revert to the original number of engine revolutions. Particularly in the case of an inexperience driver, the problem arises of the operations for reducing and increasing engine revolutions being repeatedly performed. Also, even if an experienced driver is able to grasp whether the engine has excess capacity from the travel state or the like, consideration is not given to the supply of operating oil to hydraulically-actuated devices. Thus, a reduction in engine revolutions gives rise to the possibility of work using hydraulically-actuated devices no longer being carried out smoothly due to being unable to ensure sufficient supply of operating oil.